1. Field of the Invention
This invention relates to disk drive suspensions, and more particularly, to improvements in the manufacture of disk drive suspension interconnects to secure better electrical grounding of suspension components such as copper circuit layers to grounded portions of the suspension such as stainless steel layers including stainless steel layers per se and copper metallized stainless steel layers, to enable increased numbers of copper circuit layers, and further relates to suspension products thus manufactured. The invention further relates to a resilient flying lead and flying lead terminus for disk drive suspensions. Still further, the invention relates to low impedance, low crosstalk signal traces for a disk drive suspension circuit.
2. Description of the Related Art
Suspension assemblies in hard disk drives (HDDs) include a head gimbal assembly (HGA). The HGA typically includes a gimbal assembly, a head assembly, and an interconnect assembly. The head assembly includes a highly sensitive read/write transducer, commonly referred to as a head, which is integral to the air bearing slider. The suspension assembly positions the head assembly at a generally constant distance from the surface of a rotating magnetic disk to allow the read/write transducer to read data from and write data to the magnetic disk.
The head assembly includes electrical terminals configured for interconnection to the interconnect assembly for receiving and relaying data signals. The interconnect assembly includes a plurality of transmission elements, such as wires or traces, for transmitting data to and from a magnetoresistive transducer (MR or read/write head) on the distal end of the head assembly. Amplifying circuits connected at the terminal end of the HGA process, send and receive the data signals to and from the MR head. Transmitted signals are carried between the amplifiers and the MR head by conductors formed along the suspension.
Disk drive suspension interconnects, such as Integrated Lead Suspensions (ILS) for hard disk drives typically have three layers, including a stainless steel foil that provides mechanical properties for the suspension, two or more conductive traces comprising gold plated, patterned copper conductive circuits paths that provide the electrical connection between the read/write head slider and the termination pads of the suspension, and a dielectric layer that provides electrical insulation between the stainless steel foil and the conductive traces.
In the microelectronic arts, there are two widely known methods for forming conductors on a substrate: the subtractive process and the additive process. The subtractive process utilizes a pre-formed laminate made of layers of insulative and conductive materials. A desired pattern of conductive traces is formed on the laminate by subtracting unwanted portions of the layers using etching and masking techniques. In an additive or build-up process, insulative and conductive layers are formed on a substrate, and a desired pattern of conductive traces are defined on the layers using plating or ion implantation techniques. Additive methods generally provide higher precision vias and improvements in trace density.
It is known to be desirable to ground various components of a disk drive suspension such as the body of the read/write head slider. One of the major challenges in the design of hard disk drive suspensions is attaining a suitable, reliable grounding connection between the conductive copper traces connected electrically to the slider and the underlying stainless steel foil layer given the limited space available to make the connection. The difficulty of bonding to stainless steel and dissimilarity of the metals (Cu, Au, SST) pose additional significant challenges to creating a reliable grounding of the slider, but reliable grounding is essential to maintaining the signal fidelity between the read/write head and amplifier.
Among the prior art approaches to solving the slider grounding problem is creating a hole in the dielectric between the slider and the stainless steel foil, typically 25 μm deep, and filling the hole with conductive adhesive to provide an electrical connection between the slider and the stainless steel. This approach is deficient, however, since conductive adhesive connections are typified by very high interconnect resistance resultant from the passive (self-healing) nature of the stainless steel and the lack of a conductive, fully metallic bond between the steel layer and the conductive adhesive. High interconnect resistance limits the quality of the electrical connection to ground and thus slider performance dependent on a good grounding is degraded.
Another approach to slider grounding uses a spanning lead that extends from the slider, beyond the edge of the dielectric layer and opposite the stainless steel layer where it is subsequently bent over onto the stainless steel layer and electrically and mechanically affixed there, using, typically, a conductive polymer. Spanning leads are very fragile and can be easily mis-bent during manufacture causing lowered manufacturing yields. Further, even if perfectly accomplished, the process of physically bending and adhering leads to the stainless steel suspensions is a very labor-intensive process that does not lend itself to high-volume, low-cost manufacturing, such as simultaneous gang bonding of multiple suspensions.
In both of these prior art processes the presence of conductive adhesives can cause increased drive contamination that may adversely affect drive reliability, and their use is environmentally undesirable for workers.
Additionally, it is known to provide suspension circuits having tail termination pads at the ends of the circuits, which are flying or unsupported metallic conductors. These structures, are sometimes called flying leads. One purpose of the flying lead region is to allow access to both surfaces of the conductive lead. The flying lead is typically terminated to a rigid flexible circuit on the suspension actuator using thermosonic bonding. The flying leads have metallic conductors that are unsupported by the dielectric layer that normally separates the conductive signal traces from the other conductive layers and the substrate such as stainless steel below. The flying leads therefore lack the stiffness provided by the underlying dielectric layer. U.S. Patent Publication No. 2005/0254175 by Swanson et al. shows in FIG. 2 a flying lead region 50.
Various constructions and metallurgies have been proposed for the flying leads. Swanson et al. disclose, for example, a first embodiment of a flying lead construction in FIGS. 15A-15C in which a flying lead comprises a copper signal conductor on stainless steel with nickel and gold plating, and a second embodiment in FIGS. 17A-17C in which a flying lead comprises a copper signal conductor with nickel and gold plating. U.S. Patent Publication No. 2007/0041123 by Swanson et al. discloses a flying lead portion formed of a nobel metal. U.S. Pat. No. 5,666,717 issued to Matsumoto discloses in FIGS. 1 and 2 unsupported flying leads formed by cladding (a subtractive process), sputtering, vacuum deposition, or ion plating. Matsumoto employs a conductor metal formed of copper and nickel, and overplated with a nobel metal such as gold which is resistant to corrosion and chemical etching.
During the disk drive manufacturing process, the flying leads can be used for test purposes. U.S. Pat. No. 7,110,222 issued to Erpelding describes integrated lead suspensions and tail pad terminations of those suspensions. The tail pads can be electrically connected via soldering or thermosonic bonding, both of which are widely known and practiced in the microelectronics packaging field. U.S. Patent Publication No. 2005/0254175 by Swanson et al. in FIG. 2 shows a test pad portion 46 on the side of the flying leads away from the suspension. Such a test pad portion is typically used to test the completed suspension assembly. If it is found that a read-write head, also referred to as a slider, on a suspension assembly is defective, the head must be replaced by parting the flexure tail bond and replacing the head. On the other hand if the read/write head passes its tests, typically the test pad portion 46 is cut off as no longer necessary, and the suspension is integrated into a completed disk drive unit. The fragile unsupported lead is prone to damage during assembly, test, or when separating the ultrasonic or solder terminal of this terminus for rework. In recent years, as the thickness of the copper conductor layer has decreased from about 12 μm to about 7 μm in the last few years, the lead has become even more fragile, making rework even more difficult.
Traditional methods of increasing the strength of these delicate unsupported flying leads include the use of copper alloys such as beryllium copper alloy as taught by Bennin et al. in U.S. Pat. Nos. 5,645,735 and 5,687,479, or less toxic copper alloy alternatives such as NK120 or Olin 7025 alloy. Other efforts have focused on methods of distributing the high stress on the terminations at the point of highest strain, where the unsupported leads emerge from the polyimide. An unintended drawback to increasing the toughness of the copper conductor by substituting a stronger copper alloy for use in the suspension signal traces, is that doing so undesirably increases the gimbal stiffness, whereas decreased gimbal stiffness is desired for the emerging smaller read/write heads.
The methods of Swanson et al. and Matsumoto et al. may provide a more rigid tail flying lead terminus than in prior flying leads; however, they require subsequent metal electrodeposition upon dissimilar metals. This is particularly problematic in the case of a copper conductor upon an inert steel, as the presence of the inherent passive amorphous oxide which inherently forms on stainless steel is not readily receptive to acceptable adhesion of subsequent metal deposition.
One of the many challenges in HGA design is noise reduction. For example, due to the spatial constraints inherent in microactuator design, the conductive traces formed on HDD suspensions, or flexures, cause cross talk between the read and write transmissions as signals propagate to and from the head assembly.
U.S. Pat. No. 7,342,750 issued to Yang et al. discloses an externally wireless laminated suspension for a hard disk drive for reduced noise and crosstalk. In one embodiment, the externally wireless laminated suspension has an insulating layer to electrically isolate a first and second electrical trace from a conductive support layer. The second electrical trace crosses over the first electrical trace. The first electrical trace may be made of a first part on one side of the second electrical trace and a second part on the opposite side of the electrical trace. A conductive island area may be patterned into the support layer. The conductive island area may electrically couple the first part of the first electrical trace to the second part. The number of crossover points that the first electrical trace has may equal the number of crossover points that the second electrical trace has.
The hard disk drive industry has traditionally used flexible trace pairs on flexible circuits to make electrical interconnects between the pre-amp that drives and reads the read/write signals and the read/write transducer head. In order to achieve optimal signal transmission between the pre-amp and the head, circuit impedance of the differential pairs have been designed to match the impedance of the heads and the pre-amp, which were in the 50-100 ohm range. The impedance was achieved using a circuit construction that featured the “plus” and the “minus” signals within the differential pair extending in parallel and over a stainless steel ground plane separated from the ground plane by a thin insulative layer such as a 10-20 μm thick polyimide layer. For designs that required a high impedance, such as greater than 100 ohms, the stainless steel layer could be removed under the traces and the trace width could be adjusted to match the target impedance. For low impedance trace pairs, i.e., less than 100 ohms, the stainless steel was used to improve coupling between the trace pair, thus lowering the impedance of the structure. For low data rate (i.e., <1 Gbit/sec) this construction worked well; however, as hard drive data rates continue to increases, parasitic losses in the stainless steel become an issue that limits the circuit bandwidth.
In order to achieve circuits of impedance of lower than about 50 ohms and having high bandwidths of 1 GHz or higher, suspension designs have been modified to remove the stainless steel from underneath the signal traces, and the traces have been made very wide (100-200 μm wide) with a very narrow spacing (15-30 μm) between the traces that make up the trace pair. This structure minimized the coupling to the stainless steel ground plane below, which had been causing losses leading to low bandwidth performance; however, in order to achieve lower impedance the width of the traces had to be increased. With very wide traces (150-200 μm) the impedance could be lowered down to 50 ohms with bandwidths of up to 4-5 GHz. Continuing to increase trace widths beyond about 150 μm provides minimal reduction in impedance since the increased width provides minimal increase in coupling between the trace pairs. Some of the newer read/write heads have ultra low impedances of approximately 10-30 ohms, necessitating suspension circuits having even lower impedances than those of the past. U.S. Pat. No. 5,717,547 issued to Young discloses an integrated transmission line array of multiple interleaved trace conductors symmetrically formed in a single plane for electrically interconnecting a read element or a write element of a dual element read/write head to a preamplifier circuit in a disk drive.
U.S. Pat. No. 6,762,913 issued to Even et al. discloses a method of adjusting the common mode impedance while enabling maintenance of the differential mode impedance of a pair of traces located with respect to a ground plane formed by a load beam or trace assembly of a disk drive head suspension. The method includes forming a ground plane having apertures with isolated conductive islands in the apertures for setting a desired common mode impedance. The method includes a cut and try approach using sample coupons to adjust the ratio of backed area to island area to adjust the common mode impedance while maintaining the differential mode impedance by maintaining the ratio of unbacked area to the sum of the backed and island areas.